ORIGINAL PAPER High genetic connectivity among morphologically differentiated populations of the black sea urchin Arbacia lixula (Echinoidea: Arbacioida) across the central African Mediterranean coast Temim Deli 1 & Ahmed Ben Mohamed 1 & Mohamed Hbib Ben Attia 1 & Rym Zitari-Chatti 1 & Khaled Said 1 & Noureddine Chatti 1 Received: 31 December 2015 /Revised: 16 November 2017 /Accepted: 8 December 2017 # Senckenberg Gesellschaft für Naturforschung and Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract The present study represents the first population structure analysis of the black sea urchin Arbacia lixula across part of its African Mediterranean distribution range, the Tunisian coast. This location is appropriate for the study of biogeographical processes given the presence of a well-known discontinuous biogeographic area (the Siculo-Tunisian Strait). Patterns of morphological and genetic variation in this highly dispersive echinoid species were assessed among its populations from the western and eastern Mediterranean coasts of Tunisia. A total of 109 specimens from six sites were collected and examined for morphometric variability in seven morphometric traits. Concordant results, inferred from CDA analyses, pairwise NPMANOVA comparisons and MDS plot, showed significant inter-population differences in the measured traits among the studied populations. In addition, UPGMA clustering and discriminant/Hotelling analysis enabled the delineation of two morphologically differentiated groups assigned to the western and eastern Mediterranean basins. SIMPER analysis showed that total dry weight and test diameter were major contributors to the morphometric separation between locations and among groups. Despite the extensive morphological variation found in A. lixula, genetic analysis of the mitochondrial COI marker recovered only a single evolutionary lineage and showed a lack of population structure as inferred from the results of one-level AMOVA and pairwise comparisons of genetic differentiation. Possible explanations for this genetic pattern are discussed. Notably, the lack of COI divergence patterns, highlighted by shallow genealogy associated with high haplotype diversity and low nucleotide diversity, together with a recent demographic expansion event retrieved from mismatch distribution and BSP analysis could be considered a residual effect of a recent evolutionary history of the species in the Mediterranean. Keywords Arbacia lixula . Tunisian coast . Morphological differentiation . COI . Genetic homogeneity . Demographic expansion Introduction It is generally accepted that despite the apparent connectivity of marine habitats and the high dispersal capabilities of marine organisms, populations may be organized into well-defined morphometric and genetic units (Hauser and Carvalho 2008; Hellberg 2009; Sanford and Kelly 2011; Deli et al. 2014, 2015a, 2015b). Population structure in marine species reflects the historical and contemporary interplay among a complex set of ecological, demographic, behavioral, genetic, oceano- graphic and climatic processes (Hedgecock 1986; Palumbi 1995; Benzie 1999; Bohonak 1999). If present-day processes can contribute to patterns of differentiation (Galindo et al. 2006; Gerlach et al. 2007; Selkoe et al. 2010), recognized phylogeographic barriers shared by a wide range of species should represent the main barriers to gene flow in the marine environment (Patarnello et al. 2007; Galarza et al. 2009; Kelly and Palumbi 2010). Sea urchins are benthic marine echinoderms distributed across all of the world’ s oceans (Pearse 2006). They have a Communicated by P. Martinez Arbizu * Temim Deli temimdeli@yahoo.co.uk 1 Laboratory of Genetics, Biodiversity and Enhancement of Bioresources (LR11ES41), University of Monastir, Higher Institute of Biotechnology of Monastir, Av. Tahar Hadded, B.P. 74, 5000 Monastir, Tunisia Marine Biodiversity https://doi.org/10.1007/s12526-017-0832-y